It's especially baffling since Sandisk's compatibiltiy list shows other cards in the exact same Extreme PRO SDXC UHS-II V60 line as being compatible – what is it that makes the 256GB incompatible with ...
Broadly, though, the affected models are: SanDisk Extreme PRO SDXC UHS-II V60 64GB ... with the EOS R5 II that doesn't affect the 512GB or 1TB version? Why is a 128GB V90 card okay?